Convert grease pencil to mesh in 2.8?
First, make sure you're in Object mode, with the stroke object selected.

Nowadays, the conversion tools are in the contextual menu:


If you use Left click select, you can open this menu via right-click on the stroke object. Otherwise, you need to hit the W key.

For now, using the mesh conversion on a stroke object results to a curve object for some reason, I guess it will be fixed. If you get that issue, simply redo the same thing on the curve object.